DEAR DOCTOR K: I think I might be having panic attacks, but I’m not sure. What does a panic attack feel like?

DEAR READER: People who have never experienced a panic attack typically think it’s a mild feeling of nervousness. But in fact, it’s a lot more than that.

First of all, it usually starts very suddenly; it doesn’t slowly creep up on you. Second, it’s not mild. A panic attack causes a sudden wave of intense anxiety, apprehension, fearfulness or terror. Usually, you have a terrible sense of impending doom, or of a looming catastrophe.

You may experience a sensation of smothering and a fear of going crazy or losing control. Physical symptoms may include shortness of breath, palpitations, sweating and chest pains. The symptoms are often so severe that you may believe you are having a heart attack.

During a panic attack, you may feel dissociated from the world, or even from yourself. Some people describe this as “watching a movie of yourself.”

A panic attack usually lasts five to 30 minutes, but it can continue for as long as several hours. Panic attacks typically occur during the day, but they can also rouse you from deep sleep. They’re different from nightmares. Instead of waking up and realizing you were dreaming about something frightening — like being chased — you wake up with no memories of a dream. You are simply terrified that something bad is about to happen.

What causes panic attacks? There is considerable evidence that certain parts of the brain are activated when you feel anxious. These parts include the hypothalamus and the amygdala. Brain-imaging studies have found that these areas suddenly become activated when a person experiences a panic attack. However, that introduces a question: What makes these areas of the brain suddenly become activated?

While panic attacks may be triggered by stress, they also may start for no apparent reason. A panic attack can be a single, isolated event, or attacks may occur repeatedly. Repeated panic attacks are often triggered by a particular situation.

Sometimes panic attacks occur when you are in a situation that makes you feel endangered and unable to escape — driving over a bridge, for example, or shopping in a crowded store. Such situations can repeatedly trigger panic attacks in some people.

If you think you have experienced a panic attack, make an appointment with a mental health professional. He or she will likely recommend a combination of cognitive behavioral therapy (CBT) and medication.

CBT is a type of psychotherapy. It has two parts. Cognitive therapy helps change patterns of thinking that may prevent you from overcoming your fears. Behavioral therapy helps change your reactions in situations that trigger anxiety.

Doctors often prescribe medications known as benzodiazepines for panic attacks. These drugs help you feel mentally and physically relaxed. They are used as needed to help prevent intense worry about having another attack or about the consequences of an attack. They can also be used to relieve symptoms during a panic attack.

Dr. Komaroff is a physician and professor at Harvard Medical School. AskDoctorK.com